1. Field of the Invention
The present invention relates to a zoom lens control apparatus, zoom lens control method, program, and storage medium and, more particularly, to a zoom lens control apparatus which moves the second lens unit for correcting a focal plane along with movement of the first lens unit for performing magnification operation, a zoom lens control method applied to the zoom lens control apparatus, a program for causing a computer to execute the zoom lens control method, and a storage medium which stores the program.
2. Related Background Art
Image pickup apparatuses such as a video camera which incorporates an inner focus type lens system have been conventionally available.
FIG. 3 is a view schematically showing an inner focus type lens system.
In FIG. 3, the lens system comprises a fixed lens group 101 serving as a front lens group, a zoom lens (magnification lens) 102 serving as a lens group for performing magnification operation, an aperture stop 103, a fixed lens group 104, a focus lens 105 serving as a lens group with a focus adjustment function (focusing function), and an image pickup element 106 formed from a CCD. The focus lens 105 also has a so-called compensation function of compensating for movement of the focal plane caused by magnification operation of the zoom lens 102.
As is well known, the focus lens 105 has both the compensation function and focus adjustment function in the lens system having the arrangement shown in FIG. 3. Even with the same focal length, the position of the focus lens 105 for focusing the lens on the plane of the image pickup element 106 changes depending on the object distance. When the position of the focus lens 105 for focusing the lens on the plane of the image pickup element 106 is plotted along with changes in focal length at each object distance, characteristics as shown in FIG. 4 are obtained. FIG. 4 shows the focus lens position by a locus (curve) as a function of the focal length for each object distance (e.g., 80 cm, 3 m, or ∞). During magnification operation, a locus shown in FIG. 4 is selected in accordance with the object distance. When the zoom lens 102 is driven to change the focal length, the focus lens 105 is moved along the locus, thus realizing zooming without any blur.
In a front focus type lens system, an independent compensation lens is arranged for the magnification lens, and the magnification lens and compensation lens are coupled via a mechanical cam ring. For example, a manual zooming dial is attached to the cam ring, and the focal length is manually changed. Even if the dial is quickly changed, no blur occurs in this operation as far as the focus lens is in focus because the cam ring rotates following the dial operation and the magnification lens and compensation lens move along the cam groove of the cam ring.
In the control of the inner focus type lens system having the above-mentioned features, characteristic information about a plurality of loci shown in FIG. 4 is generally stored in some format in a lens control microcomputer. A locus is selected in accordance with the object distance, and movement of the focal plane by magnification operation is corrected along the selected locus in zooming. The position of the focus lens 105 is controlled by reading out the position of the focus lens 105 with respect to the position (focal length) of the zoom lens 102 from the lens control microcomputer. For this purpose, the performance of an actuator which drives the focus lens 105 is important. As is apparent from FIG. 4, when the zoom lens 102 moves at a constant speed or almost constant speed at the same object distance, the moving speed and moving direction of the focus lens 105 momently change. In other words, the actuator of the focus lens 105 must respond to the speed with a high precision of about 1 Hz to several hundred Hz.
An example of the actuator having this performance is a stepping motor, which is generally being used for the focus lens 105 of the inner focus lens system. The stepping motor rotates in perfect synchronism with a stepping pulse output from the lens control microcomputer or the like, and keeps the stepping angle per pulse constant. The stepping motor can, therefore, obtain a high speed response characteristic, high stop precision, and high position precision. Further, the stepping motor keeps the rotation angle constant with respect to the number of stepping pulses. The stepping pulse can be directly used as an increment type encoder, and no special position encoder needs to be added to the lens system.
To perform magnification operation while keeping an in-focus state by using the stepping motor, locus information shown in FIG. 4 must be stored in the lens control microcomputer or the like, as described above. Locus information is read out in accordance with the position or moving speed of the magnification lens, and the focus lens is moved based on the information. Alternatively, a function which expresses the position of the focus lens 105 by using the object distance and the focal length of the zoom lens 102 as variables may be adopted.
A method of performing compensation calculation between loci and calculating the standard moving speed of the focus lens in the use of the locus data table will be explained.
FIG. 5 is a graph showing an example of the locus characteristic used in a conventional locus tracing method applied when magnification operation is executed while an in-focus state is maintained using a stepping motor. The locus characteristic is stored in the lens control microcomputer.
In FIG. 5, Z0, Z1, Z2, . . . , Z6 represent positions of the magnification lens (zoom lens); and a0, a1, a2, . . . , a6 and b0, b1, b2, . . . , b6, typical loci indicating focus positions for respective object distances. p0, p1, p2, . . . , p6 represent intermediate loci each calculated based on the two loci, and are calculated by
p(n+1)=|p(n)xe2x88x92a(n)|/|b(n)xe2x88x92a(n)|xc3x97|b(n+1)xe2x88x92a(n+1)|+a(n+1)xe2x80x83xe2x80x83(1) 
For example, a point p1 is calculated by equation (1), a ratio with which a point p0 internally divides a line segment (b0xe2x88x92a0) is obtained, and a point which internally divides a line segment (b1xe2x88x92a1) in accordance with this ratio is set as p1. When the focus lens 105 is located at the point p0, the standard moving speed of the focus lens 105 for maintaining an in-focus state can be obtained from the position difference (p1xe2x88x92p0) between the points p1 and p0, and the time taken to move the zoom lens 102 from the position Z0 to the position Z1.
Calculation of the position of the focus lens 105 when the stop position of the zoom lens 102 is not a position (zoom boundary position) on stored typical locus data will be explained.
FIG. 6 is a locus graph for explaining an interpolation method in the direction of the magnification lens position. Some of data in FIG. 5 are extracted, and magnification lens position data are arbitrarily set.
In FIG. 6, the ordinate and abscissa respectively represent the positions of the focus lens 105 and zoom lens 102. Typical locus positions (discrete positions of the focus lens 105 for discrete positions of the zoom lens 102) stored in the lens control microcomputer are represented by zoom lens positions Z0, Z1, . . . , Zkxe2x88x921, Zk, . . . , Zn, and focus lens positions a0, a1, . . . akxe2x88x921, ak, . . . , an and b0, b1, . . . , bkxe2x88x921, bk, . . . , bn for respective object distances at the zoom lens positions Z0, Z1, . . . , Zkxe2x88x921, Zk, . . . , Zn. Assume that the zoom lens position is an intermediate position Zx which is not a discrete position (zoom boundary position) on locus data, and ax and bx represent focus lens positions on locus data for respective object distances at the intermediate position Zx. The focus lens positions ax and bx are calculated by
ax=akxe2x88x92(Zkxe2x88x92Zx)xc3x97(akxe2x88x92akxe2x88x921)/(Zkxe2x88x92Zkxe2x88x921)xe2x80x83xe2x80x83(2) 
bx=bkxe2x88x92(Zkxe2x88x92Zx)xc3x97(bkxe2x88x92bkxe2x88x921)/(Zkxe2x88x92Zkxe2x88x921)xe2x80x83xe2x80x83(3) 
That is, ax and bx can be calculated by internally dividing line segments between data for the same object distances among four stored typical locus data (ak, akxe2x88x921, bk, and bkxe2x88x921) in accordance with an internal ratio obtained from the current magnification lens position Zx and the two sandwiching zoom boundary positions (Zkxe2x88x921 and Zk).
Let p0, p1, . . . pkxe2x88x921, px, pk, . . . , pn be focus lens positions (loci) between the focus lens positions (loci) a0, a1, . . . , akxe2x88x921, ax, ak, . . . , an and the focus lens positions (loci) b0, b1, . . . , bkxe2x88x921, bx, bk, . . . , bn. Then, pk and pkxe2x88x921 can be calculated by internally dividing line segments between data for the same focal lengths among the four stored typical data (ak, akxe2x88x921, bk, and bkxe2x88x921) in accordance with an internal ratio obtained from ax, px, and bx, as given by equation (1). In zooming from the wide-angle end to the tele-photo end, the moving speed of the focus lens 105 for maintaining an in-focus state can be attained from the position difference between a tracing destination focus position pk and a current focus position px, and a time taken for the zoom lens 102 to move from Zx to Zk. In zooming from the tele-photo end to the wide-angle end, the standard moving speed of the focus lens 105 for maintaining an in-focus state can be attained from the position difference between a tracing destination focus position pkxe2x88x921 and a current focus position px, and a time taken for the zoom lens 102 to move from Zx to Zkxe2x88x921. This locus tracing method has been invented.
When the zoom lens 102 moves from the tele-photo end to the wide-angle end, an in-focus state can be maintained by the above-described locus tracing method because varying loci converge in this direction, as is apparent from FIG. 4. However, when the zoom lens 102 moves from the wide-angle end to the tele-photo end, an in-focus state cannot be maintained by the same locus tracing method because which of loci is traced by the zoom lens 102 located at a convergence point is not known.
FIGS. 7A and 7B are graphs for explaining a conventional locus tracing method invented to solve the above problem. In FIGS. 7A and 7B, the abscissa represents the zoom lens position. The ordinates in FIGS. 7A and 7B represent the AF evaluation signal level and focus lens position, respectively. An AF evaluation signal is a sharpness signal representing the in-focus degree, and is formed from the high-frequency component of an image signal.
In FIGS. 7A and 7B, the in-focus cam locus in zooming in on a given object to be picked up is a locus 604. In this case, the standard moving speed of tracing the in-focus cam locus on a wide-angle (W) side from a zoom position 606 (Z14) is positive (move toward the in close end of the focus lens), and the standard moving speed of tracing the in-focus cam locus on a tele-photo (T) side from the zoom position 606 (Z14) is negative (move toward the infinite end of the focus lens). When the focus lens 105 traces the cam locus 604 while completely maintaining an in-focus state, the magnitude of the AF evaluation signal maintains a maximum value 601. In general, the AF evaluation signal level has an almost constant value in zooming which maintains an in-focus state.
In FIG. 7B, let Vf0 be the standard moving speed of the focus lens which traces the in-focus cam locus 604 in zooming, and Vf be the actual moving speed of the focus lens 105. If zooming is done by increasing/decreasing the focus lens moving speed Vf from the standard focus lens moving speed Vf0, the focus lens 105 traces a zigzag locus 605. Then, the AF evaluation signal level shown in FIG. 7A repeats a peak and valley, as represented by a curve 603. The AF evaluation signal level curve 603 exhibits the maximum value 601 at positions (Z0, Z2, Z4, . . . , Z16) where the loci 604 and 605 cross each other, and a minimum value 602 at positions (Z1, Z3, Z5, . . . , Z15) where the moving vector of the locus 605 changes its direction.
If a level TH1 corresponding to the minimum value 602 is set as a switching point, and the moving vector of the locus 605 is switched every time the AF evaluation signal level reaches the level TH1, the switched moving direction of the focus lens 105 can be set to a direction close to the in-focus locus 604. That is, zooming which suppresses any blur can be achieved by controlling the moving direction and speed of the focus lens 105 so as to reduce the blur every time the image blurs by the difference between the maximum value 601 and the minimum value 602 (TH1) of the AF evaluation signal level.
In zooming from the wide-angle end to the tele-photo end in which the cam locus as shown in FIG. 4 diverges from convergence by the above-described method, the focus lens moving speed Vf is controlled with respect to the standard moving speed (calculated using p(n+1) obtained by equation (1)) though the speed not is optimal for the object distance, unlike the standard moving speed Vf0 at which an in-focus state is maintained. At the same time, switching operation is repeated along the locus 605 in accordance with changes in AF evaluation signal level. As a result, a locus along which the AF evaluation signal level does not become lower than the minimum value 602 (TH1), i.e., a locus which does not generate any blur more than a predetermined amount can be selected. Appropriately setting the level TH1 enables zooming which reduces the blur amount so the user may not visually recognize the blur.
Letting Vf+ be the positive correction speed and Vfxe2x88x92 be the negative correction speed, the moving speed Vf of the focus lens 105 is given by
Vf=Vf0+Vf+xe2x80x83xe2x80x83(4) 
Vf=Vf0+Vfxe2x88x92xe2x80x83xe2x80x83(5) 
These correction speeds Vf+ and Vfxe2x88x92 are determined such that the internal angle defined by two vectors of the moving speed Vf obtained by equations (4) and (5) is divided into two by the vector of the standard focus lens moving speed Vf0. This prevents any offset when a locus to be traced is selected by the zooming method. There is also proposed a method of changing the correction amount represented by the correction speed in accordance with an object to be to be picked up, the object distance, and the depth of field, changing the increase/decrease period of the AF evaluation signal, and thus improving the tracing locus selection precision.
In the control of the above-described magnification operation, an in-focus state is detected using an image signal from the image pickup element 106. The magnification operation control process is generally performed in synchronism with a vertical sync signal when this lens system is mounted in a video camera.
FIG. 8 is a flow chart showing conventional lens drive process procedures executed in the lens control microcomputer. This process will be explained along steps.
In step S701, the RAM and various ports in the microcomputer are initialized.
Step S702 is a routine for performing communication with a system control microcomputer (to be referred to as a xe2x80x9csystem computerxe2x80x9d hereinafter) which controls the operation system of the camera main body. The lens control microcomputer receives, from the system computer, input information from a zoom SW unit operated by the operator. The lens control microcomputer transfers, to the system computer, information representing that zoom operation is in progress, and magnification operation information such as the zoom lens position. These pieces of information are displayed to the operator on a display or the like.
Step S703 is an AF process routine in which an automatic focus adjustment process is done in accordance with changes in AF evaluation signal.
Step S704 is a zoom process routine in which a compensation operation process for maintaining an in-focus state is performed in magnification operation. The standard drive direction and standard drive speed of the zoom lens 102 which traces a cam locus as shown in FIG. 5 are calculated by the above-mentioned method.
Step S705 is a selection routine of the drive direction and speed. Of the drive directions and drive speeds of the zoom lens 102 and focus lens 105 calculated in the process routines of steps S703 and S704, drive directions and drive speeds to be used are selected in accordance with AF operation, magnification operation, or the like. The lens is also set not to be driven toward the tele-photo side from the tele-photo end, the wide-angle side from the wide-angle end, the in close side from the in close end, and the infinity side from the infinite end. The tele-photo end, wide-angle end, in close end, and infinite end are set by software so as to prevent each lens from colliding with the movable end portion of the lens mechanism.
FIG. 9 is a graph showing an example of each end of the focus lens set by software.
For example, the in close photographing distance is 1 cm at the wide (W) end, and ensures 1 m through the total focal length. As typical loci stored in the lens control microcomputer, a locus 801 for an object distance of 5 mm and a locus 802 for an object distance of 90 cm are prepared. The 5-mm locus 801 is set as an in close end within a focal length range of a position 804 to a position 805. A fixed value 807 equal to the most in close position on the 90-cm locus 802 is set as an in close end within a focal length range of the position 805 to a position 806. The 90-cm locus 802 is set as an in close end within a focal length range on the tele-photo side from the position 806.
As for the infinite end, a locus 808 separated by a predetermined amount from an infinite locus 803 to the infinite direction is obtained and set as an infinite end.
In order to enable focusing at a photographing distance from the infinite end to the in close end, the in close end and infinite end of the focus lens must be set outside a range determined by the above in close end and infinite end. This is because, to focus the lens on a main object at infinity, the focus lens must be driven to the superinfinite side to confirm that the AF evaluation signal level decreases from the infinity-position level. On the in close side, the focus lens 105 is moved to an object distance of 90 cm over the position of a main object at, e.g., an object distance of 1 m. Only after it is confirmed that the AF evaluation signal level decreases, the lens can be focused to an object distance of 1 m. When the depth of field or the focal depth increases in accordance with the aperture state, the focal length range from the in close end to infinite end of the focus lens is set wider than a focusable photographing distance in order to facilitate focus adjustment. Further, the focal length range from the in close end to the infinite end is set wide in consideration of a blur which may be generated by expansion/contraction of the lens barrel upon changes in the ambient temperature or the like.
Referring back to FIG. 8, in step S706, drive/stop of each lens is controlled by outputting a control signal to the motor driver in accordance with the drive direction/drive speed information of the zoom lens and focus lens selected in step S705.
After the process of step S706 ends, the flow returns to step S702. A series of processes shown in FIG. 8 are executed in synchronism with a vertical sync period in a video camera when the lens system is mounted in the video camera. That is, the process starts in response to input of a vertical sync signal in the process of step S702.
When, however, the locus tracing method described with reference to FIGS. 7A and 7B is applied to the conventional inner focus type lens system, the AF evaluation signal varies due to movement of an object, the camera work, hand shake, or the like in zooming operation which refers to the AF evaluation signal. As a result, the correction speed direction may not be properly switched. In this case, locus tracing may be done in a direction apart from the in-focus locus.
Especially when the correction speed is added toward the infinite position in zooming in on an object at the infinite position, the lens is focused to a superinfinite position over the infinite position. Since the infinite end is set deep, as described above, the blur amount becomes large until the lens is reversely moved toward the in close end. The image quality provided to the photographer via the viewfinder or monitor becomes low.
When the focus lens is located at an in close position from the infinite in-focus position by a predetermined amount (near-focus state), only an object at the infinite position within the frame blurs, and an object at another position hardly blurs. To the contrary, when the focus lens falls within a superinfinite region on the superinfinite side from the infinite end by a predetermined amount, all objects within the frame blur. In this superinfinite region, the AF evaluation signal level abruptly decreases, and the in-focus direction cannot be accurately determined. It takes a long time for the focus lens to return to the infinite object position, giving the photographer a greatly blurred impression.
The present invention has been made to overcome the conventional drawbacks, and has as its object to provide a zoom lens control apparatus, zoom lens control method, program, and storage medium capable of preventing any blur in magnification operation and providing a comfortable photographing environment.
According to a feature of the present invention, a zoom lens control apparatus which has a first lens unit for executing magnification operation, and a second lens unit for correcting variations in image plane along with magnification operation of the first lens unit and serving as a focusing function, and drives the second lens unit on the basis of a focus adjustment signal comprises a first setting circuit which sets a first moving range for the second lens unit along with movement of the first lens unit, and a second setting circuit which sets a second moving range different from the moving range for the second lens unit along with movement of the first lens unit.
In particular, the second moving range is narrower than the first moving range, the second moving range is set in zooming from a wide-angle side to a tele-photo side, and the first moving range is set in zooming from the tele-photo side to the wide-angle side.
Further, the second moving range is set when the second lens unit is driven based on the focus adjustment signal in zooming from the wide-angle side to the tele-photo side, and the first moving range is set when the second lens unit is driven not based on the focus adjustment signal.
The first moving range is wider over an infinite object than a region surrounded by a moving range of the second lens unit when a focus is adjusted from an in close object to the infinite object, and the second moving range is closer to the infinite object than the first moving range and is narrower.
The first and second setting circuits include microcomputers.
In addition, the present invention is characterized by a program which controls the above-described apparatus. The above and other objects, features, and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments in conjunction with the accompanying drawings.